So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure contains four quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles would be the critical parts from the thyristor, allowing it to control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their working status. Therefore, thyristors are commonly used in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any silicon-controlled rectifier is generally represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The working condition from the thyristor is the fact that when a forward voltage is used, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage can be used between the anode and cathode (the anode is attached to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), as well as the indicator light will not glow. This demonstrates that the thyristor will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used to the control electrode (known as a trigger, as well as the applied voltage is called trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is switched on, whether or not the voltage on the control electrode is removed (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At this time, in order to shut down the conductive thyristor, the power supply Ea must be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used to the control electrode, a reverse voltage is used between the anode and cathode, as well as the indicator light will not glow at the moment. This demonstrates that the thyristor will not be conducting and may reverse blocking.
- In conclusion
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct if the gate is exposed to a forward voltage. At this time, the thyristor is within the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will stay switched on no matter the gate voltage. Which is, after the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for your thyristor to conduct is the fact that a forward voltage needs to be applied between the anode as well as the cathode, plus an appropriate forward voltage ought to be applied between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode made from three PN junctions. It could be equivalently viewed as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is used between the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still switched off because BG1 has no base current. If a forward voltage is used to the control electrode at the moment, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will likely be introduced the collector of BG2. This current is brought to BG1 for amplification and after that brought to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A big current appears in the emitters of the two transistors, which is, the anode and cathode from the thyristor (the size of the current is really dependant on the size of the load and the size of Ea), so the thyristor is entirely switched on. This conduction process is done in a very limited time.
- After the thyristor is switched on, its conductive state will likely be maintained from the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to decrease the anode current so that it is insufficient to keep up the positive feedback process. The best way to decrease the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to keep the thyristor in the conducting state is called the holding current from the thyristor. Therefore, as it happens, so long as the anode current is less than the holding current, the thyristor may be switched off.
What is the distinction between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of any transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current on the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mainly utilized in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method of working
The transistor controls the collector current by holding the base current to attain current amplification.
The thyristor is switched on or off by managing the trigger voltage from the control electrode to realize the switching function.
The circuit parameters of thyristors are related to stability and reliability and usually have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications sometimes, because of their different structures and working principles, they have noticeable variations in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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